Poly(phosphoric acid) (PPA)-Promoted 5- exo -Cyclization of Iminium Ions Generated in Situ: A Facile Access to Functionalized Indene Derivatives

Article abstract of DOI:10.1055/s-0036-1589009

A metal-free Bronsted acid promoted two-component reaction between cinnamaldehydes and sulfonamides is described. This cascade process provides a simple and atom-economical alternative synthesis of a range of functionalized indenes from easily available starting materials. The resulting N -indenylsulfonamides were readily converted into the corresponding indenylenamines or indanones.

Full text of DOI:10.1055/s-0036-1589009

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–G
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A
en
Y.-F. Zhu et al.
Letter
Synlett
Poly(phosphoric acid) (PPA)-Promoted 5-exo-Cyclization of Imi-
nium Ions Generated In Situ: A Facile Access to Functionalized
Indene Derivatives
R1
R1
R1
Yi-Fan Zhua
TsNH2
SmI2
R3
R2
R3
R2
Xin-Le Genga
Yong-Hong Guana
Wei Tenga
O
R3
R2
cat. PPA
toluene
THF–H2O
NHTs
O
R1, R2 = alkyl, aryl
16 examples, up to 99% yield
R
3 = alkyl, MeO, Cl, F, etc.
Xiaohui Fan*a,b,c
· metal-free
· easily available starting materials
· single-step operation
· formation of two bonds and one ring
a School of Chemical and Biological Engineering, Lanzhou
Jiaotong University, Lanzhou 730070, P. R. of China
fanxh@mail.lzjtu.cn
b Beijing National Laboratory for Molecular Sciences (BN-
LMS), College of Chemistry, Peking University, Beijing
100871, P. R. of China
c YMU-HKBU Joint Laboratory of Traditional Natural
Medicine, Yunnan Minzu University, Kunming 650500,
P. R. of China
Received: 21.01.2017
Accepted after revision: 26.03.2017
Published online: 04.05.2017
R1+
N
R1
+N
DOI: 10.1055/s-0036-1589009; Art ID: st-2017-w0060-l
R2
R3
R2
R3
endo-mode
Abstract A metal-free Brønsted acid promoted two-component reac-
tion between cinnamaldehydes and sulfonamides is described. This cas-
cade process provides a simple and atom-economical alternative syn-
thesis of a range of functionalized indenes from easily available starting
materials. The resulting N-indenylsulfonamides were readily converted
into the corresponding indenylenamines or indanones.
exo-mode
Figure 1 Cyclization modes of iminium ions
al materials, and transition-metal complexes4 that attract
continuing interest from synthetic chemists. A variety of
methods, including inter- and intramolecular reactions, as
well as ring-expansion and ring-contraction processes, have
been developed to access various functionalized indenes.5
In 2005, Takai and co-workers reported a rhenium-cata-
lyzed [3+2] reaction of imines with alkynes to give indenyl-
amine derivatives (Scheme 1),6 which are promising build-
ing blocks in biological materials and medicines.7 Following
this pioneering work, several other advances were report-
ed.8 In 2009, Wang, Lu, and their co-workers developed an-
other strategy for preparing substituted indenylamines by a
cascade reaction of aziridines and propargyl alcohols, in
which iminium ion cyclization plays a major role (Scheme
1).9 Recently, our group reported an FeCl3-catalyzed two-
component reaction for the synthesis of indenylamine de-
rivatives through cyclization of iminium ions generated in
situ.10 This method features a high efficiency in the genera-
tion of two new bonds and one five-membered ring in a
single operation from readily accessible substrates. Howev-
er, the reaction is limited to electron-rich substituted cin-
namaldehydes. As a part of our ongoing program on C–
O/C=O bond-functionalization reactions,11 we report a met-
al-free poly(phosphoric acid) (PPA)-promoted procedure for
Key words indenes, iminium ions, indanones, exocyclization, poly-
phosphoric acid, cascade reaction
The iminium ion based cyclization reaction is among
the most important methods for the construction of cyclic
systems, as it permits multiple transformations to take
place in one step, and makes synthetic routes shorter and
more efficient with substantial reductions in waste and
time. Consequently, many protocols based on this reaction
have been developed for the synthesis of complex struc-
tures. However, most of these are focused on the formation
of nitrogen-containing heterocycles by an endo mode of cy-
clization (Figure 1).1 One well-known transformation of this
type is the Pictet–Spengler reaction, which has played a
major role in organic synthesis for over a hundred years.2 In
contrast, the exo mode of cyclization has received limited
attention, although it provides a unique opportunity to de-
velop novel transformations for the efficient construction of
carbocyclic skeletons.3
Functionalized indenes are an attractive family of car-
bocyclic compounds, and functionalized indene moieties
are present in many biologically active molecules, function-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–G

B
Y.-F. Zhu et al.
Letter
Synlett
mentally benign Brønsted acid that has shown good cata-
lytic ability in the Pictet–Spengler reaction.1a We found that
the reaction medium critically affects the efficiency of this
reaction. No reaction occurred in MeNO2 or EtOH as the sol-
vent (Table 1, entries 1 and 2), whereas in DCE or MeCN, the
cyclization product 3a was obtained in 9% and 15% isolated
yield, respectively (entries 3 and 4). Gratifyingly, when the
reaction was carried out in toluene (entry 5), a 91% yield of
3a exclusively was obtained, with full conversion of 1a. In-
creasing the reaction temperature or the catalyst loading
had no obvious effects on the reaction (entry 6). However,
lowering the amount of PPA from 20% to 15 mol% was clear-
ly unfavorable for the reaction, as the yield of product 3a
decreased markedly to 34% (entry 7). On replacing PPA with
H3PO4, only a trace of 3a was obtained, along with unreact-
ed 1a and a small amount of the corresponding imine (en-
try 8). Other Brønsted acids such as HCl or H2SO4 were also
found to be unsuitable for use as catalysts in this reaction
(entries 9 and 10).
Takai, 2005
N
tBu
H
NHtBu
Ph (R2)
2 (Ph)
R2
Ph
cat. [ReBr(CO)3(thf)]2
toluene, reflux, 24 h
+
R
Wang and Lu, 2009
Ph
Ph
cat.
Yb(OTf)3
Ts
HO
Ph
Ph
+
N
TsN
LA
Ph
DCE
80 °C
12 h
Ph
Ph
NHTs
Ph
Ph
Ph
Fan, 2014
R1
R1
R1
R2
TsNH2
R2
NHTs
O
cat. FeCl3
DCE
40–60 °C
R2
NHTs
Scheme 1 Protocols for the formation of indenylamines
Having established the optimal reaction conditions (Ta-
ble 1, entry 5), we treated a range of aldehydes 1 with
TsNH2 to determine the scope and limitations of this PPA-
promoted reaction (Table 2).12
the synthesis of various indenylamine derivatives, which
permits the use of either electron-rich or electron-deficient
cinnamaldehydes as suitable substrates.
At the outset of this investigation, we focused our atten-
tion on the reaction of (2E)-2-methyl-3-phenylacrylalde-
hyde (1a, 1.0 equiv) with TsNH2 (2a, 1.2 equiv) in the pres-
ence of 20 mol% of PPA as a catalyst in a range of solvents, to
establish the optimal reaction conditions (Table 1). PPA was
chosen as the catalyst because it is a cheap and environ-
The results show that a wide range of arylated acroleins
are effective substrates in this system, furnishing the corre-
sponding cyclization products in moderate to excellent
yields (Table 2, entries 1–15). Generally, the reaction was
facilitated by aldehydes that contain an electron-rich arene
π-nucleophile. The introduction of an electron-withdraw-
ing group, such as a chloro or fluoro group, on the phenyl
ring resulted in moderate yields of the corresponding cy-
clization products (entries 14 and 15). Note that electron-
deficient cinnamaldehydes such as 1n failed to react with
TsNH2 in our previous work when FeCl3 was used as the cat-
alyst;10 the present catalytic system is therefore more effi-
cient with electron-deficient aldehydes. Additionally, me-
thoxy-substituted aldehydes also gave better yields com-
pared with those in our previous work (entries 5 and 6);
the difference in the coordination abilities of PPA and FeCl3
is probably responsible for this result. Interestingly, when
1- and 2-naphthyl-substituted acroleins 1g and 1h were
used as substrates, the reaction occurred exclusively at the
C-2 and C-1 positions through a 5-exo ring closure to give
the benzene-fused indene-type products 3g and 3h, re-
spectively, in high yields, without the formation of C-8 or C-
3 ring-closure products (entries 7 and 8). The structure of
the skeleton of N-indenylsulfonamide 3h was determined
by an X-ray single-crystal analysis (Figure 2).13
Table 1 Optimization of the Reaction Conditionsa
catalyst
O
TsNH2
+
solvent, 80 °C, 12 h
NHTs
1a
2a
3a
Entry
Catalyst (mol%)
Solvent
Yieldb (%)
1
2
PPA (20)
PPA (20)
PPA (20)
PPA (20)
PPA (20)
PPA (30)
PPA (15)
H3PO4 (20)
HCl (20)
H2SO4 (20)
MeNO2
EtOH
0
0
3
DCE
9
4
MeCN
15
91
92
34
trace
0
5
toluene
toluene
toluene
toluene
toluene
toluene
6
7
To our disappointment, cinnamic ketones such as (3E)-
3-methyl-4-phenylbut-3-en-2-one failed to react, even un-
der harsh conditions, presumably because of the lower
electrophilicity of the carbonyl group of the ketone. Note
that the presence of an α-substituent in the aldehyde sub-
strate is crucial to this transformation; upon subjecting (E)-
cinnamaldehyde or (E)-3-phenylbut-2-enal (1p) to this re-
8
9
10
trace
a Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), solvent (3 mL), 80 °C,
12 h.
b Isolated yield.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–G

C
Y.-F. Zhu et al.
Letter
Synlett
action, only the corresponding imines, along with the unre-
acted aldehydes, were obtained (Table 2, entry 16). 3,3-Di-
phenylacrylaldehyde (1m), which lacks an α-substituent,
did, however, react (entry 13), presumably owing to its spe-
cial structure in which the carbonyl group and one of the
phenyl rings are oriented in a cis-fashion. These results are
consistent with our previous work.10
Figure 2 X-ray crystal structure of N-indenylsulfonamide 3h
a
Table 2 Reaction of Various Aldehydes with TsNH2
Entry
1
Substrate
Product
Temp (°C)
80
Time (h)
12
Yield (%)b
91
O
1a
3a
NHTs
O
2
3
1b
1c
3b
3c
80
80
16
16
81
86
NHTs
O
O
NHTs
Ph
NHTs
4
5
1d
1e
3d
3e
80
80
16
18
99
79
Ph
O
O
O
O
O
NHTs
6
1f
3f
80
24
90
O
NHTs
7
8
1g
1h
3g
3h
80
80
24
24
90
91
O
NHTs
O
NHTs
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–G

D
Y.-F. Zhu et al.
Letter
Synlett
Table 2 (continued)
Entry
Substrate
Product
Temp (°C)
40
Time (h)
48
Yield (%)b
9
1i
3i
82
Ph
NHTs
O
Ph
O
Ph
NHTs
10
11
1j
3j
40
40
48
48
73
82
Ph
O
1k
3k
Et
Et
NHTs
12
13
1l
3l
80
80
16
16
80
75
O
NHTs
Ph
Ph
1m
3m
O
NHTs
O
14
15
1n
1o
3n
3o
80
80
16
16
64
61
Cl
F
Cl
NHTs
O
F
NHTs
16
1p
3p
80
16
0
O
NHTs
a Reaction conditions: aldehyde 1 (0.2 mmol), TsNH2 (0.24 mmol), solvent (3 mL).
b Isolated yield.
Next, we examined the scope of the amine by using al-
dehyde 1a as the substrate (Scheme 2). Benzenesulfon-
amide (2b) and 4-chlorobenzenesulfonamide (2c) reacted
smoothly with aldehyde 1a to give the desired N-indenyl-
sulfonamides 3ab and 3ac in 87% and 83% yield, respective-
ly. However, aniline (2d), 4-nitroaniline (2e), and piperidine
(2f) failed to react. With methanesulfonamide (2g) or benz-
amide (2h), the corresponding cyclization products 3ag and
3ah were obtained along with the corresponding imines
and inseparable mixtures.
PPA (20 mol%)
toluene
O
+
RNH2
NHR
3ab–ah
1a
2b–h
2b PhSO2NH2
87% (80 °C, 12 h)
83% (80 °C, 12 h)
0% (100 °C, 12 h)
0% (100 °C, 12 h)
0% (100 °C, 12 h)
5% (100 °C, 24 h)
2c 4-Cl-PhSO2NH2
2d PhNH2
2e 4-O2NC6H4NH2
2f piperidine
2g MeSO2NH2
Subsequently, the derivation of the N-indenylsulfon-
amide 3a was carefully investigated (Scheme 3). Initial at-
tempts to remove the N-tosyl group under a variety of con-
ditions14 (sodium naphthalide, Mg/MeOH, SmI2, LAH/THF,
2h PhCONH2
15% (100 °C, 24 h)
Scheme 2 Reaction of aldehyde 1a with various amines
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–G

E
Y.-F. Zhu et al.
Letter
Synlett
or NaOH/MeOH) failed to afford the deprotected product.
However, on treatment with Mg/MeOH, 3a was trans-
formed into the indenylenamine 4a in almost quantitative
yield. Interestingly, treatment of 3a with SmI2, pyrrolidine,
and water15 in THF gave indanone 5a in 91% yield without
the formation of the detosylation product (Scheme 3); oth-
er indenylamines such as 3l also underwent this transfor-
mation smoothly under similar conditions.16 The reason for
this transformation is not clear at present,17 and further ex-
perimental and mechanistic studies are underway in our
group.
PPA (20 mol%)
toluene
NTs
NHTs
6a
1a
3a: 92% (80 °C / 24 h)
PPA (20 mol%)
toluene
O
OH
0% (80 °C / 12 h)
Scheme 4 Control experiments
conditions
onance-stabilized cationic intermediate B (a hybrid of the
resonance structures of iminium ion E and allylic cation F).
Intermediate B can be transformed into the corresponding
imine 6a or its isomer C (a hybrid of resonance structures of
G and H) through loss of a proton or a bond-rotation proce-
dure, respectively. Subsequently, intermediate C undergoes
an intramolecular aza-Friedel–Crafts reaction (or 4π-elec-
trocyclization) followed by loss of a proton to give product
3a. In this ring-closure reaction, isomerization of the cat-
ionic intermediate B to its geometrical isomer C is neces-
sary before cyclization can occur. This process is facilitated
by the cinnamaldehyde having an α-substituent, because
this induces an A1,2 strain between the α-substituent and
the aromatic group. Additionally, the generation of the key
intermediates B and C also benefits from the presence of an
electron-donating group at the α-position. Indeed, (2Z)-2-
bromo-3-phenylacrylaldehyde is completely inert to this
transformation, even under harsh reaction conditions.
conditions A: Mg, MeOH 99% yield
NHTs
NHTs
NHTs
conditions B: TBAF, THF 80% yield
3a
3a
4a
SmI2, pyrrolidine, H2O
THF, r.t.
O
5a, 91%
SmI2, pyrrolidine, H2O
THF, r.t.
O
NHTs
3l
5l, 88%
Scheme 3 Derivation experiments
Next, we examined the transformation of sulfonamide
3a into the corresponding indenone under the reported
conditions using tetrabutylammonium fluoride (TBAF) or
I2/K2CO3.8a,9 However, none of the desired product was ob-
tained under these conditions. Treatment of sulfonamide
3a with I2/K2CO3 led to an intractable mixture, whereas on
treatment with TBAF, 3a was converted into the indenyl-
enamine 4a in 80% yield (Scheme 3).
H
O
O
PPA
O
1a
TsNH2
+
N H
H
Me
Ts
With the aim of understanding the mechanism, two
control experiments were conducted (Scheme 4). First, on
treatment with 20 mol% of PPA in toluene at 80 °C, the pre-
viously prepared aldimine 6a was smoothly converted into
sulfonamide 3a in 92% isolated yield. Note that at the begin-
ning of this reaction a proportion of 6a decomposed into al-
dehyde 1a and TsNH2, but these disappeared by the time
the reaction was complete. Secondly, reaction of 1a under
the optimized conditions for 12 hours without addition of
TsNH2 resulted in no cyclization product being obtained.
The reaction is therefore unlikely to involve a cycliza-
tion/substitution process.
On the basis of the above results and previous stud-
ies,10,18 we propose the tentative reaction pathway shown
in Scheme 5. Initially, nucleophilic addition of TsNH2 to the
carbonyl group of the aldehyde 1a with the aid of PPA gives
the amino alcohol intermediate A, which is dehydrated af-
ter protonation of the hydroxy group by PPA to afford a res-
OH
NHTs
– H+
H+
PPA
NHTs
Ph
NTs
Me
– H2O
– PPAO–
Me
Me
6a
A
B
Me
PPAO–
Me
NHTs
3a
– PPA
NHTs
H
C
D
O
P
O
NHTs
NHTs
PPA =
B
HO
O
P
OH
O–
n
n
Me
Me
OH
OH
E
F
O
O
Me
NHTs
Me
C
PPAO
HO
P
O
P
=
NHTs
OH
OH
G
H
Scheme 5 Tentative reaction pathway
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–G

F
Y.-F. Zhu et al.
Letter
Synlett
In summary, we have developed a novel cascade ap-
proach to the synthesis of functionalized indene derivatives
by condensation of substituted cinnamaldehydes with sul-
fonamides. Electron-rich and electron-deficient cinnamal-
dehydes are both suitable as substrates. This Brønsted acid
catalyzed two-component reaction has some indubitable
advantages, such as the ready availability of the starting
materials and catalyst, the simplicity of the metal-free pro-
cedure, and its high atom-economy. Additionally, we found
that the resulting indenylamines can be transformed into
the corresponding indanones by treatment with SmI2, pyr-
rolidine, and water; this transformation has not been re-
ported previously and is expected to find widespread use in
related reactions. Further studies on these reactions are in
progress in our group.
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Funding Information
National Natural Science Foundation of China (21162013),
(21562030)
Beijing National Laboratory for Molecular Sciences (20140133)
Acknowledgment
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We thank Professor Zhi-Xiang Yu for helpful discussions.
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1589009.
S
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p
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Diness, F.; Meldal, M. Curr. Opin. Drug Discovery Dev. 2003, 6,
801. (f) Laine, A. E.; Lood, C.; Koskinen, A. M. P. Molecules 2014,
19, 1544.
(3) (a) Yu, X.; Lu, X. Adv. Synth. Catal. 2011, 353, 569. (b) Yu, X.; Lu,
X. Tetrahedron Lett. 2011, 52, 2076. (c) Li, Y.; Xu, M.-H. Asian J.
Org. Chem. 2013, 2, 50. (d) Zhou, S.-L.; Li, J.-L.; Dong, L.; Chen, Y.-
C. Org. Lett. 2011, 13, 5874. (e) Wang, S.-G.; Zhang, W.; You, S.-L.
Org. Lett. 2013, 15, 1488. (f) Chihab-Eddine, A.; Daїch, A.; Jilale,
A.; Decroix, B. Tetrahedron Lett. 2001, 42, 573. (g) Mecozzi, T.;
Petrini, M.; Profeta, R. Tetrahedron: Asymmetry 2003, 14, 1171.
(12) N-(1H-Inden-1-yl)benzenesulfonamides 3a–p; General Pro-
cedure
TsNH2 (0.24 mmol) and PPA (20 mol%) were added to a stirred
solution of aldehyde 1 (0.20 mmol) in toluene (2 mL), and the
resulting mixture was stirred at 40–80 °C until the aldehyde
was completely consumed (TLC). The reaction was then
quenched by addition of sat. aq NaHCO3 (3 mL), and the mixture
was extracted with EtOAc (3 × 5 mL). The organic layers were
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–G

G
Y.-F. Zhu et al.
Letter
Synlett
combined, washed with brine, dried (Mg2SO4), and filtered. The
solvent was removed in vacuo, and the residue was purified by
column chromatography [silica gel, PE–EtOAc (10:1)].
4-Methyl-N-(2-methyl-1H-inden-1-yl)benzenesulfonamide
(3a)
Chem. Soc. 2012, 134, 4941. (d) Schultz, E. E.; Sarpong, R. J. Am.
Chem. Soc. 2013, 135, 4696. (e) Jiang, B.; Yang, C.-G.; Wang, J.
J. Org. Chem. 2001, 66, 4865.
(15) Ankner, T.; Hilmersson, G. Org. Lett. 2009, 11, 503.
(16) 2-Methylindan-1-one (5a); Typical Procedure
1
White solid; yield: 55 mg (91%); mp 130–132 °C. H NMR (400
To a solution of SmI2 (0.13 M in THF; 16 mL, 2.1 mmol) was
added 3a (0.52 mmol, 155.6 mg) followed by H2O (15.6 mmol,
0.28 mL) and pyrrolidine (10.4 mmol, 0.86 mL) under an argon
atmosphere. The mixture immediately turned white upon addi-
tion of pyrrolidine. The mixture was then stirred for 0.5 h at r.t.,
then diluted with EtOAc (6 mL) and treated with 0.5 M aq HCl (4
mL). The aqueous phase was extracted with EtOAc (2 ×), and the
organic phases were combined, washed with brine, dried (Na2-
SO4), filtered, and concentrated. The crude product was purified
by column chromatography [silica gel, PE–EtOAc (20:1)] to give
a yellow oil; yield: 69 mg (91%). 1H NMR (600 MHz, CDCl3):
δ = 7.76 (d, J = 7.7 Hz, 1 H), 7.61–7.56 (m, 1 H), 7.45 (d, J = 7.7
Hz, 1 H), 7.37 (t, J = 7.4 Hz, 1 H), 3.44–3.36 (m, 1 H), 2.79–2.66
(m, 2 H), 1.32 (t, J = 7.0 Hz, 3 H). 13C NMR (151 MHz, CDCl3):
δ = 209.4, 153.4, 136.4, 134.6, 127.3, 126.5, 124.0, 42.0, 35.0,
16.3. HRMS (EI): m/z [M+] calcd for C10H10O: 146.0726; found:
146.0725.
MHz, CDCl3): δ = 7.88 (d, J = 8.2 Hz, 2 H), 7.37–7.33 (m, 2 H),
7.18–7.16 (m, 2 H), 7.07 (m, 1 H), 6.97 (m, 1 H), 6.32 (s, 1 H),
4.70 (d, J = 9.5 Hz, 1 H), 4.54 (d, J = 9.5 Hz, 1 H), 2.47 (s, 3 H), 1.89
(s, 3 H). 13C NMR (100 MHz, CDCl3): δ = 145.9, 143.5, 143.3,
143.1, 138.4, 129.7, 128.3, 127.7, 127.1, 124.8, 123.2, 120.2,
62.3, 21.5, 13.7. HRMS (ESI): m/z [M
17H21N2O2S: 317.1318; found: 317.1315.
+
NH4]+ calcd for
C
N-(2,3-Dimethyl-1H-inden-1-yl)-4-methylbenzenesulfon-
amide (3l)
1
White solid; yield: 50 mg (80%); mp 175–176 °C. H NMR (400
MHz, CDCl3): δ = 7.89 (d, J = 8.2 Hz, 2 H), 7.38 (d, J = 8.0 Hz, 2 H),
7.26–7.20 (m, 1 H), 7.08 (d, J = 7.4 Hz, 1 H), 7.00 (t, J = 7.4 Hz, 1
H), 6.77 (d, J = 7.3 Hz, 1 H), 4.69 (d, J = 9.2 Hz, 1 H), 4.44 (d, J =
9.2 Hz, 1 H), 2.48 (s, 3 H), 1.94 (s, 3 H), 1.81 (s, 3 H). 13C NMR
(100 MHz, CDCl3): δ = 145.2, 143.5, 142.8, 138.7, 137.9, 134.0,
129.8, 128.3, 127.3, 125.0, 123.0, 118.3, 62.4, 21.6, 11.1, 10.3.
HRMS (ESI): m/z [M + Na]+ calcd for C18H19NNaO2S : 336.1029;
found: 336.1035.
(17) (a) Zheng, H.-X.; Xiao, Z.-F.; Yao, C.-Z.; Li, Q.-Q.; Ning, X.-S.;
Kang, Y.-B.; Tang, Y. Org. Lett. 2015, 17, 6102. (b) Szostak, M.;
Spain, M.; Procter, D. J. Chem. Soc. Rev. 2013, 42, 9155.
(c) Ouyang, K.; Hao, W.; Zhang, W.-X.; Xi, Z. Chem. Rev. 2015,
115, 12045.
(13) CCDC 965333 contains the supplementary crystallographic
data for 3h. The data can be obtained free of charge from
The Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/getstructures.
(18) (a) Reid, J. P.; Simón, L.; Goodman, J. M. Acc. Chem. Res. 2016, 49,
1029. (b) So, Y.-H.; Heeschen, J. P. J. Org. Chem. 1997, 62, 3552.
(14) (a) Knowles, H. S.; Parsons, A. F.; Pettifer, R. M.; Rickling, S. Tet-
rahedron 2000, 56, 979. (b) Moon, B.; Han, S.; Yoon, Y.; Kwon, H.
Org. Lett. 2005, 7, 1031. (c) Trost, B. M.; Silverman, S. M. J. Am.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–G